1. Field of the Invention
The invention relates to loudspeakers and more particularly, but not exclusively, the invention relates to vibration exciters for exciting resonance in resonant panel-form loudspeakers e.g. of the general kind described in our published International patent application WO97/09842 and which have become known as ‘distributed mode’ loudspeakers.
2. Description of the Related Art
A known form of exciter used to drive a distributed mode loudspeaker panel is based on converting an electrical input into a force which is applied normal to the panel surface. This generates bending waves which emanate from the drive point. By suitably positioning this point on the loudspeaker panel, the modes in the panel can be coupled with sufficient density to make the panel act as a loudspeaker.
A disadvantage of this method of panel excitation is that it is usually preferable for the force to be applied near to the central portion of the panel, which would, for example, be impractical for a transparent panel, used in association with a visual display, where the vibration exciter should not be visible.
Bending waves derived from a typical force exciter also cause whole body (i.e. timpanic) mode, whose radiated sound field may interfere with a boundary placed parallel to, and in close proximity with, the rear of the panel, to form a cavity. With such a cavity behind a panel the whole body mode may appear at an undesirably high frequency. This limits the low frequency range of the loudspeaker, and may also result in an excessive resonance or peak in the frequency response at the dominating coupled system resonance.
It is an object of the invention to provide a method and means for exciting a resonant loudspeaker panel near to an edge of the panel.
It is another object of the invention to provide a method and means for exciting a resonant loudspeaker panel which will reduce the excitation of whole body modes.